The present invention relates generally to antenna calibration, and more particularly to absolute calibration of global navigation satellite system antennas.
Global navigation satellite systems (GNSS), such as GPS (U.S.A.) and GLONASS (Russia), are well known in the art. A navigation receiver receives and processes radio signals transmitted by satellites located within line-of-sight distance of the receivers. The satellite signals comprise carrier signals that are modulated by pseudo-random binary codes. The receiver measures the time delay of the received signal relative to a local reference clock or oscillator. These measurements enable the receiver to determine the so-called pseudo-ranges between the receiver and the satellites. The pseudo-ranges are different from the ranges (distances) between the receiver and the satellites due to various noise sources and variations in the time scales of the satellites and receiver. If the number of satellites is large enough and satellite clock differences are known, then the measured pseudo-ranges can be processed to determine the user location and coordinate time scales. Some satellites broadcast more than one code-modulated carrier signal, such as a GPS satellite which broadcasts a carrier in the L1 frequency band and another carrier in the L2 frequency band. More than two frequency carriers are possible.
The requirement of determining user location with a high degree of precision, and the desire to improve the stability and reliability of measurements, have led to the development of differential navigation (DN). In differential navigation, the task of finding the user position, also called the Rover, is performed relative to a Base station (Base). The precise coordinates of the Base station are assumed known and the Base station is generally stationary during measurements. The Base station has a navigation receiver which receives and processes the signals of the satellites to generate measurements. These signal measurements are transmitted to the Rover via a communication channel (e.g., wireless). The Rover uses these measurements received from the Base, along with its own measurements taken with its own navigation receiver, in order to determine its location precisely.
The location determination accuracy of differential navigation may be further improved by supplementing the pseudo-range measurements with measurements of the phases of the satellite carrier signals. If the carrier phase of the signal received from a satellite in the Base receiver is measured and compared to the carrier phase of the same satellite measured in the Rover receiver, measurement accuracy may be obtained to within several percent of the carrier's wavelength.
The collection of GPS pseudorange (code) and phase (carrier) data at two or more sites simultaneously and subsequent processing to determine precise relative coordinates (baselines) is well known in the art. Using well-known techniques, code and phase data are usually processed in either one-way (no differencing), single-, double-, or triple-difference modes. It is well known that one-way, single- and double-differences of carrier phase measurements have biases that must be estimated. In particular, if successful, the integer nature of the double-difference biases (also called ambiguities) will be recognized and the solution may be constrained so that the integer nature of the double-difference ambiguities is preserved.
A GPS solution for a baseline provides the vector between the phase centers (or electrical centers) of the antennas at either end of the baseline. However, antennas do not have single well defined phase centers, but instead the location of an antenna phase center is a function of the direction from which the antenna receives a signal. In order to take full advantage of the millimeter level carrier phase measurements, these variations in antenna response, or range (distance) corrections, as a function of satellite azimuth and elevation must be modeled.
Initially, such models treated the antenna corrections as an offset to the range measurement to a theoretical phase center, whereby the phase center coordinates were given relative to a recognized physical antenna reference point known as the Antenna Reference Point (ARP).
Originally, antenna phase center calibrations were available only in relative form. It was recognized that two antennas of the same type, behaving (theoretically) identically to incoming signals because of their proximity to each other, would yield no information about the actual response to incoming voltage fronts. However, if the two antennas responded differently, e.g., using two different antenna designs, the differences in measured response could be determined. As such, original antenna phase center calibrations were calculated in terms relative to a given reference antenna. The United States Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Geodetic Survey, has used this relative calibration technique to calibrate various types of antennas, using the Jet Propulsion Laboratory (JPL) Dorne/Margolin choke ring antenna type T (JPL D/M+crT) as the reference antenna. These calibrations do not provide absolute phase calibration for the tested antennas, but rather the relative calibrations with respect to this particular reference antenna.
While the above described technique utilizes relative calibration, the absolute nature of antenna response variations has also been studied. In particular, it has been recognized that if a short baseline with antennas of the same or different design are used, but one antenna is rotated with known rotation angles, the one being rotated responds in different ways depending on the angular orientation, and thus the absolute rather than relative responses can be estimated.
There are two types of absolute antenna calibration, one using antennas of the same type, and the other using antennas of different types. In order to determine absolute response using two of the same type antennas at different ends of a baseline, differences in response to different orientations from the different ends of the baseline can be used to model absolute response. Alternatively, if the antennas at each end of the baseline are of different types, then each would respond differently even without changing orientations. Thus, one of the antenna's absolute responses must be known so that the other antenna's responses at the opposite end of the baseline can be determined. Once a known antenna's response has been removed (measurements corrected or moved to the ARP), the unknown response of the other antenna can be recovered should sufficient samples be collected.
While the above described calibration techniques are useful, there are certain disadvantages. First, there is significant reliance on the consistency of antenna types. Using the relative calibration technique, consistency between various reference antennas (e.g., JPL D/M+crT) is required. Similarly, using the absolute calibration techniques with the same antenna type, the accuracy of the calibration relies on the fact that the two antennas of the same type will react in the same way. Given manufacturing variations, such reliance may result in inaccurate calibrations. Another disadvantage to both the relative and absolute methods is that the precise baseline between the reference and test antennas must be known so that variations can be determined from a study of measurement residuals after the known baseline contribution is removed. This requires that precise baseline measurements be made prior to antenna calibration.
What is needed is an antenna calibration technique which can accurately perform absolute antenna calibrations without reliance on a precisely known baseline, and without the need for precise advance information of the characteristics of the reference antenna.